Currently, charcot reconstruction surgery is based on a combination of internal and external fixation of a patient's damaged foot. The specific type of internal fixation usually revolves around the use of large diameter, cannulated hip screws—typically 6.5 mm diameter. These screws are often made out of titanium. Other metals may be used, but titanium is usually chosen because of its biocompatibility.
The problems that may occur with known methods of internal fixation include the following:
1. Charcot joints often fail to fuse; therefore, the screws, instead of compressing arthodesis sites, end up acting as load bearing members as long as the patient lives. Because of this, there exists a significant screw failure rate. That is, the screws may simply break, usually at the thread runout at the end of the screw where it resides in the midtarsal bones. The bending moments are the highest at the midtarsal joints exceeding 120,000 PSI; hence, the screw metal is insufficient for a long term 2× safety factor with commonly-used 6.5 mm titanium screws.
2. The heads of commercially available screws can tear the head of a patient's metatarsal bone, often causing infection and creating a need to remove the screw, resulting in poor outcome.
3. Similarly, the lateral column of the foot requisitely needs to be stabilized in charcot, to correct and maintain subluxation/dislocation at the articulation between the metatarsal base 4/5 and the cuboid. The dislocation of this lateral column with the cuboid coming down through the bottom of the foot can cause an essentially untreatable plantar lateral diabetic ulcer. The screws currently commercially available do not take into account the anatomy of the flaring metatarsal bases. Also they can have a head design that could potentially wedge itself into place locking this column.
4. No current screw has any surface on it that can grow into the bone, making it a true load sharing device with the bone. Instead, the screws often loosen and become a concern for infection.